Creative Commons and RDF

As early as fall 2001, CC had settled on the approach of creating machine-readable licenses based on the World Wide Web Consortium’s then-emerging Resource Description Framework (RDF), part of the W3C Semantic Web Activity.⁵

The motivation for choosing RDF in 2001, and for continuing to use it now, is strongly connected to the CC vision: promoting scholarly and cultural progress by making it easy for people to share their creations and to collaborate by building on each other’s work. In order to lower barriers to collaboration, it is important that the machine expression of licensing information and other metadata be interoperable. Interoperability here means not only that different programs can read particular metadata properties, but also that vocabularies—sets of related properties—can evolve and be extended. This should be possible in such a way that innovation can proceed in a distributed fashion in different communities—authors, musicians, photographers, cinematographers, biologists, geologists, and so on—so that licensing terms can be devised by local communities for types of works not yet envisioned. It is also important that potential extensions be backward compatible: existing tools should not be disrupted when new properties are added. If possible, existing tools should even be able to handle basic aspects of new properties. This is precisely the kind of

“interoperability of meaning” that RDF is designed to support.

5 The Semantic Web Activity is a large collaborative effort led by the W3C aimed at extending the Web to become a universal medium for data exchange, for programs as well as people. See http://www.w3.org/2001/sw.

1.1.1  RDF triples

RDF is a framework for describing entities on the Web. It provides exceptionally strong support for interoperability and extensibility. All entities in RDF are named using a simple, distributed, globally addressable scheme already well known to Web users: the URL, and its generalisation the URI.⁶

For example, Lawrence Lessig’s blog, a document identified by its URL http://lessig.org/blog, is licensed under the CC Attribution license.

That license is also a document, identified by its own URL http://

creativecommons.org/licenses/by/3.0. The property of “being licensed under”, which we will call “license” can itself be considered a Web object and identified by a URL. This URL is http://www.w3.org/1999/xhtml/

vocab#license, which refers to a webpage that contains information describing the “license” property. This particular webpage, maintained by the Web Consortium, is the reference document that describes the vocabulary.⁷

Instantiating properties as URLs enables anyone to use those properties to formulate descriptions, or to discover detailed information about an existing property by consulting the page at the URL, or to make new properties available simply by publishing the URLs that describe those properties.

As a case in point, CC originally defined its own “license” property, which it published at http://creativecommons.org/ns#license since no other group had defined in RDF the concept of a copyright license.

When the XHTML Working Group introduced its own license property in 2005, we opted to start using their version, rather than maintain our own CC-dependent notion of license. We were then able to declare that http://creativecommons.org/ns#license is equivalent to the new property http://www.w3.org/1999/xhtml/vocab#license, simply by updating the description at http://creativecommons.org/ns#license. Importantly, RDF makes this equivalence interpretable by programs, not just humans, so that

“old” RDF license declarations can be automatically interpreted using the new vocabulary.

In general, atomic RDF descriptions are called triples. Each triple consists of a subject, a property, and a value for that property of the subject. The triple

6 The term URI (universal resource identifier) is a generalisation of URL (universal resource locator). While a URL refers in principle to a resource on the Web, a URI can designate anything named with this universal hierarchical naming scheme. This generality is used in CC REL for items such as downloaded media files.

7 The vocabulary is also referenced in http://www.w3.org/2011/rdfa-context/rdfa-1.1.html, which sets default vocabulary prefixes and terms for RDFa.

that describes the license for Lessig’s blog could be represented graphically as shown in Figure 1: a point (the subject) labelled with the blog URL; a second point (the value) labelled with the license URL; and an arrow (the property) labelled with the URL that describes the meaning of the term

“license”, running from the blog to the license. In general, an RDF model, as a collection of triples, can be visualized as a graph of relations among elements, where the edges and vertices are all labelled using URLs.

Figure 1: An RDF Triple represented as an edge between two nodes of a graph

1.1.2  Expressing RDF as text

Abstract RDF graphs can be expressed textually in various ways. One commonly used notation, RDF/XML, uses XML syntax. In RDF/XML the triple describing the licensing of Lessig’s blog is denoted:

<rdf:RDF xmlns:rdf=”http://www.w3.org/1999/02/22-rdf-syntax-ns#”

xmlns:xhtml=”http://www.w3.org/1999/xhtml/vocab#”>

<rdf:Description rdf:about=”http://www.lessig.org/blog/”>

<xhtml:license rdf:resource=”http://creativecommons.org/

licenses/by/3.0/” />

</rdf:Description>

</rdf:RDF>

One desirable feature of RDF/XML notation is that it is completely self-contained: all identifiers are fully qualified URLs. On the other hand, RDF/XML notation is extremely verbose, making it cumbersome for people to read and write, especially if no shorthand conventions are used. Even this

http://www.w3.org/1999/xhtml/vocab#license

http://lessig.org/blog/

<rdf:RDF xmlns:rdf=”http://www.w3.org/1999/02/22-rdf-syntax-ns#”

xmlns:xhtml=”http://www.w3.org/1999/xhtml/vocab#”>

<rdf:Description rdf:about=”http://lessig.org/blog”>

<xhtml:license resource=”http://creativecommons.org/licenses/by/3.0/” />

</rdf:Description>

</rdf:RDF>

@prefix xhtml: <http://www.w3.org/1999/xhtml/vocab#> .

<http://lessig.org/blog> xhtml:license <http://creativecommons.org/licenses/by/3.0/> . http://creativecommons.org /licenses/by/3.0/

simple example (verbose as it is) uses a shorthand mechanism: the second line of the description beginning xmlns:xhtml defines “xhtml:” to be an abbreviation for http://www.w3.org/1999/xhtml/vocab#, thus expressing the license property in its shorter form, xhtml:license, on the fourth line.

Since the introduction of RDF, the Web Consortium has developed more compact alternative syntaxes for RDF graphs. For example the N3 syntax would denote the above triple more concisely:⁸ <http://lessig.org/blog>

<http://www.w3.org/1999/xhtml#license>

<http://creativecommons.org/licenses/by/3.0/> .

We could also rewrite this using a shorthand as in the RDF/XHTML example above, defining: xhtml: as an abbreviation for http://www.w3.org/1999/

xhtml/vocab#:

@prefix xhtml: <http://www.w3.org/1999/xhtml/vocab#> .

<http://lessig.org/blog/> xhtml:license

<http://creativecommons.org/licenses/by/3.0/> .

The shorthand does not provide improved compactness or readability if a prefix is only used once as above. In N3, prefixes are typically defined only when they are used more than once, for example to express multiple properties taken from the same vocabulary. In RDF/XML, because of the stricter parsing rules of XML, there is a bit less flexibility: predicates can only be expressed using the shorthand, while subjects can only be expressed using the full URI.

1.2  CC’s previous recommendation: RDF/XML in HTML